1. Field of Invention
This invention relates to methods and devices for the formation and attachment of conductive balls to a substrate, and more specifically to methods and devices for forming and attaching conductive balls to substrates used in semiconductor devices.
2. Discussion of Prior Art
As semiconductor devices have become larger and more complex, the number of input and output ports on the devices has increased significantly. With that increase has come a need for more leads on the packages that house complex semiconductor devices. As lead count has grown, it has been necessary to reduce the spacing between adjacent leads to conserve space and to preserve the performance of the devices. The resultant packages with fine pitch leads present significant problems in handling and placement. To alleviate those problems, a new package type, known as the ball grid array (BGA) has been developed. On BGA packages, the fine, peripherally attached, flying leads of conventional surface mount packages are replaced with an array of conductive balls. Typically those conductive balls are composed of solder and attached to one surface of the BGA package substrate. The resultant packages are considerably more robust and significantly easier to mount properly than conventional surface mount packages.
All current methods of attaching solder balls to BGA packages start with discrete spheres of solder of precisely controlled volume. The most common diameter for these spheres is 0.75 mm and the number of balls on a given package can be from around one hundred to several thousand. Current BGA packages are most common with 169 to 540 balls.
The prior art offers several methods of positioning and attaching solder balls to a ball grid array substrate. All involve positioning discrete solder spheres on individual, solder ball receiving areas on the substrate. The solder ball receiving areas are typically conductive pads with a solder wettable surface. The solder balls are then melted and the molten solder forms a metallurgical joint between the pad and the solder. The first challenge to these procedures is to position one and only one solder ball on each pad, and to do so at minimum cost. While there are several methods for distributing the solder balls to their proper locations, all of these processes encounter a second challenge; that of keeping hundreds of very small spheres in their proper location until they can be heated to melt the solder. Accomplishing that is understandably difficult and the consequences of failure is a costly rework in the best case and loss of the entire device in the worst case. In most of these methods, a rosin type solder flux is used to temporarily tack the solder balls to the pads. Because of the relatively weak adhesive quality of the flux, it is still easy to dislodge a ball after it has once been properly positioned. Balls displaced after initial proper placement most commonly become conjoined with a neighboring ball. The result is one or more pads that have solder balls with two or more times as much solder as intended and one or more pads that have no solder ball at all. Because of the susceptibility of these methods to fail to supply a ball for every pad, and the vulnerability to subsequent displacement of the balls, a very reliable inspection system is essential for identifying packages with displaced balls for rework.
There are two common methods employed to position discrete balls on package substrates. One involves using a mask with a pattern of holes in the mask that corresponds to the pattern of solder ball receiving areas on the substrate. The combination of the mask thickness and the hole diameter is such that only one solder ball can occupy a hole at a time such that a resident ball in a given hole is securely captured as long as the bottom of the mask is closed by the package substrate. The process involves applying solder flux to the pads, positioning the mask over the pads on the package substrate, filling each of the holes with a solder ball and then lifting the mask straight up to leave a ball on each pad. While the mask method requires little capital equipment, it is slow and highly labor intensive. This method is also the most prone to missing balls due to the lack of a method of positively capturing the solder balls.
The other common method of positioning solder balls is taught in U.S. Pat. No. 4,871,110, issued to Fukasawa et al on Oct. 3, 1989. An improvement to that method is taught in U.S. Pat. No. 5,284,287 issued to Wilson et al on Feb. 8, 1994. Both patents teach the use of a vacuum tool to capture individual balls, one for each of the pads, and then transferring those balls to the pads on the BGA substrate. Wilson adds a method of adding sticky solder flux to the balls after they have been captured by the vacuum tool and before they are transferred to the pads on the substrate. Use of a vacuum to capture and hold the solder balls does reduce the probability of missing balls but does not completely eliminate the problem of missing balls. The need to apply solder flux that can temporarily maintain the position of the solder balls complicates the process of applying flux, and as practiced by Wilson, introduces the potential for displacing balls from the vacuum tool during the fluxing operation. The vacuum methods are faster and more automatable than the mask methods but also require significantly more capital equipment and tooling. While the vacuum methods are faster, the process steps must be carried out very deliberately and there are rate constraining steps during the solder ball capture and the flux application steps that limit the ultimate speed of the process.
A new product employs a third method that is similar to that taught by Reid in U.S. Pat. No. 4,2216,350, issued Aug. 6, 1980. Reid teaches the use of a non-fusible web to hold toroid shaped solder preforms to apply solder to the terminal pins of semiconductor packages which are designed to be mounted by soldering the terminal pins in the holes of printed circuit boards. The new product substitutes spherical shaped masses of solder captured in a non-fusible web. The product is designed to be positioned on the BGA substrate such that the solder spheres are directly above, or in contact with the pads on the substrate. The system is then heated to above the melting point of the solder to melt and release the solder masses, which then wet the pads and form the metallurgical bonds with the pads. While this method virtually eliminates the necessity for capital equipment for positioning the solder balls with a manual process and significantly reduces the cost of automating the process, it does not eliminate the problem of missing solder balls. The non-fusible web is relatively weak and the solder balls are held in place by friction. Any flexing of the carrier during handling is liable to displace one or more solder balls. While the non-fusible carrier does eliminate much of the capital cost of solder ball attachment, the cost of solder balls mounted in the carrier is much greater than if they were purchased in bulk as discreet balls.
Adding to the cost of all of these methods is the cost of the solder balls themselves. The value of the metal in 1,000, 0.75 mm diameter solder balls is on the order of $0.03 but the price of said balls is over 10 times that. The cost of the solder balls over the value of the metals is the result of the complexity of the manufacturing, grading and handling of such small and precise spheres.
Despite the sophistication of some of the current methods, all of the heretofore known methods for forming and attaching solder balls to ball grid array substrates suffer from a number of disadvantages:
(a) The processes most commonly used to attach solder balls to BGA packages is cumbersome and tedious and perceived to be so by most of those responsible for implementing such processes. There is the perception, held by many of those same people, that one of the keys to promoting the BGA style package to the level of a commercial success, is providing a method of ball attachment that can be more easily and more widely implemented.
(b) The processes used to position the hundreds of individual solder balls are susceptible to either omitting balls, misplacing balls or introducing extra balls. Misplaced or missing balls, if not corrected by a rework process, render the package useless. Since ball attachment is typically the last step in the packaging process, the loss of a package during the ball attachment operation also means the loss of the device in the package. Since BGA packages are used primarily for high lead count, high value devices, the failure to attach a few dimes worth of solder balls could result in the loss of a microprocessor worth hundreds of dollars.
(c) Prior art BGA substrates, which have hundreds of tiny, weakly adhering solder balls require extra care in handling as the individual balls are prone to displacement.
(d) The cost of positioning hundreds of discrete spheres is high.
(e) The cost of producing the small, precise balls of solder alloy is many times the value of the solder alloy.
(f) Inspection systems capable of identifying a small percentage of packages with missing and/or displaced solder balls virtually demands automated vision systems which are very expensive.
(g) Current solder ball attachment processes use rosin type, "sticky" solder flux to maintain the position the solder balls between the time they are initially positioned and the time they are melted and bonded to the receiving pads of the substrate. While the flux serves its purpose as a glue and is an effective flux, it is difficult to clean off after reflow and incompatible with soldering processes that use no-clean fluxes or no flux at all. Rosin type fluxes require cleaning solvents that are toxic and damaging to the ozone layer and as such are rapidly being replaced by materials and processes that do not require solvent cleaning.